False Negative Results of First-Tier Treponemal Antibodies in an HIV-Infected Patient

Abstract

Abstract Introduction/Objective Syphilis diagnosis relies on detecting Treponema pallidum antibodies in serum or CSF. In the reverse syphilis screening algorithm, an automated enzyme immunoassay first detects serum antibodies against treponemal antigens. A negative result halts further testing; however, a positive result prompts manual testing for antibodies against non-treponemal antigens such as cardiolipin. The sensitivity of first-tier automated assays is typically high. We present a case of an HIV patient who initially tested negative for syphilis in the first-tier assay but was subsequently diagnosed with the infection. Methods/Case Report A 30-year-old HIV-positive female (CD4+ count 24/mm3), previously diagnosed with chlamydia and gonorrhea, experienced blurred vision, eye pain, fever, and a palmoplantar rash. Despite clinical suspicion of syphilis, the initial first-tier chemiluminescence immunoassay (CIA) for treponemal antibodies (LIAISON Treponema, DiaSorin, Stillwater, MN) returned a negative result, precluding any further syphilis testing at that time. However, a skin biopsy revealed interface dermatitis and spirochetes. CSF analysis showed pleocytosis, increased proteins, and borderline low glucose, with both VDRL and IFA assays negative. Based on these findings, we retested for serum antibodies by repeating the CIA test and performing RPR and TP-PA assays outside the algorithm sequence. Despite reconfirmation of the CIA negativity, both RPR (titer 1:64) and TP-PA were positive, leading to a secondary syphilis diagnosis and penicillin G treatment. Subsequent investigations aimed to explain the initial false negative CIA result. Serial dilution of the patient's serum up to 1:1000 did not alter the result, excluding the 'hook effect' caused by an excessive concentration of antibodies. Further serum testing at a different laboratory using another type of first-tier immunoassay yielded a positive result, pointing toward differences in the composition of treponemal antigens utilized across various manufacturer assays as the likely cause of the discrepancy. Results (if a Case Study enter NA) NA Conclusion Given their high sensitivity, automated first-tier treponemal assays rarely yield false negative results, whereas false positives are the most common ‘lab error’ and are typically resolved with second-tier assays. Nevertheless, this case highlights that false negatives can occur and, thus, stresses the need to re-evaluate the approach in diagnosing syphilis, especially in patients with antibody production abnormalities, such as those with HIV.